There are various types of fuel cells, for example, the alkaline type, the phosphoric acid type, the molten carbonate type, the solid electrolyte type, the solid polymer type, etc. Among such fuel cells, the alkaline fuel cell is generally constructed of an anode electrode, a cathode electrode, and an electrolyte solution that separates the two electrodes from each other. The anode electrode is supplied with hydrogen as a fuel (or with a fuel that is used as a material for producing hydrogen), and the cathode electrode is supplied with oxygen (or atmospheric air or the like). At the anode electrode, the hydrogen supplied thereto becomes hydrogen atoms, which react with hydroxide ions that reach the anode electrode from the cathode electrode by permeating through the electrolyte solution, so that water is produced. The electrons generated by this reaction are sent to an external electrode. At the cathode electrode, on the other hand, as the electrons extracted at the anode electrode are supplied via an external circuit, oxygen molecules adsorbed in the cathode electrode receive the electrons, and react with water of the electrolyte solution to produce hydroxide ions.
For example, in conjunction with an alkaline fuel cell disclosed in Japanese Patent Application Publication No. 2002-8706. (JP-A-2002-8706), a system for supplying hydrogen to the anode electrode is disclosed in which a reaction chamber is provided on the anode electrode side and pure hydrogen is generated. The interior of the reaction chamber is filled with a potassium hydroxide aqueous solution. During the operation of the fuel cell, silicon is supplied into the reaction chamber, in which silicon and potassium hydroxide react to produce pure hydrogen. The hydrogen thus produced is supplied to the anode electrode, so that the above-described electrochemical reaction occurs.
In some cases, for effective utilization of fuel, the fuel supplied to the anode electrode is used by circulating it. However, in the alkaline fuel cell, hydrogen and oxygen react at the anode electrode to produce water. Therefore, in the case where the anode electrode is supplied with a water-soluble fuel by circulation, fuel and water sometimes mix and the concentration of the fuel gradually declines. However, it is preferable that the fuel at a certain level of concentration be supplied in order to maintain high performance in power generation.
To this end, for example, a method in which the fuel concentration is detected, and at the time point when the detected fuel concentration declines to a certain level, the fuel is discharged and new fuel is supplied is conceivable. However, in some cases, the exhaust fuel contains unreacted fuel. From the viewpoint of effective utilization of fuel, it is preferable that such a wasteful discharge of fuel be restrained.